Wiring harnesses are used in a wide variety of environments, including small scale equipment such as personal computers, domestic appliances such as washing machines and tumble dryers, automobiles, and aircraft ranging from helicopters to large passenger aircraft and transport planes. A wiring harness includes a number of elements including wires used to conduct data signals or power, which are bound together by for example helically wound tape to produce a compact and easy to manage array. The wires will be accessible at various points in the harness, in accordance with the positions of items in the equipment with which they are to connect. The harness is usually configured in three dimensions, so that it can adapt to the contours of the equipment in which it is to be located, and the positions of items to which wires are to be connected. The harness may include additional features such as heat resistant portions. The harness design will usually include elements in the form of components such as terminal blocks, connectors, clips, grommets and tubes, as wells as labels, tags and sleeves used to distinguish between wires. A harness may also include other items as well as electrical wires, such as washer fluid tubes used in automobiles. The expression “wiring harness” used in this specification is not restricted to electrical data or power wires.
The design of a wiring harness involves two basic aspects, namely the electrical logic which defines the elements required and how they are to be connected, and the three dimensional layout which deals with the shape of the equipment in which the harness is to be used, the actual physical positions of components to which wires in the harness are to be connected, and so forth. Various computer aided design packages can deal with these aspects, either separately or together. There is a known software system which enables input regarding the electrical and three dimensional requirements to be converted into data concerning the material requirements for the wiring harness. Such software is available, for example, from Mentor Graphics Corporation. This software allows users to view harness plans, conduct design validation exercises, and to produce wire and component requirements, cost estimates, purchasing schedules, sales quotes, formboard drawings, wire cutting details, testing routines, and data for automated assembly systems.
In the automobile industry, in particular, it has become normal to offer a range of options to a customer. For example, there may be a choice of gasoline or diesel engines, manual or automatic transmissions, air conditioning options, electrically operated windows and sunroofs, anti-lock brakes and traction control, in-car entertainment options and so forth. In the past, these may have been offered in predetermined configurations, such as a base model and one or two luxury models with different combinations of features. In such circumstances it was feasible to provide a specific harness for each model, if it would be sold in high enough numbers, or to provide a harness capable of handling all of the possible requirements so that, in the lower models, various wires and components would be redundant. However, in recent years the trend has been for automobile manufacturers to provide a large range of options which, with a limited number of exceptions, can be combined together in any way the customer desires. This presents the possibility of having to produce an enormous number of harness designs, each for a particular combination of options, or a complex universal harness which can cope with all possibilities and will have many redundant wires and components. The use of a universal harness will add considerably to weight and expense, and is no longer considered feasible by, for example, those in the automobile and aviation industries.
A system which has been introduced to deal with the problem of managing large numbers of option combinations, is the “composite harness”, again available from Mentor Graphics Corporation. The composite harness system involves the use of a master harness design, with derivatives which have various of the available options. In the design process, a composite harness is in some ways similar to a universal, redundant harness, the harness design containing the full wiring for all possible options. However, since there is no intention to build the composite harness as such, the wires and components belonging to the optional features are marked so that they can be included or excluded depending on the harness to be built. Thus, an automobile manufacturer would define an ideal set of packages to offer and a number of derivative or variant harnesses would be specified representing the choices available to the consumer. In practice, the number of derivatives for any composite harness design has been finite, and usually no greater than thirty. The harness manufacturer would fully decompose the design into individual harness variants which would be engineered, costed and built.
The composite harness still assumes a certain restriction of choice for the consumer. Customers increasingly expect to be able to tailor even mass produced automobiles to their own specification. Whilst in theory the composite harness technique could have dealt with the increasing demands, by specifying a new derivative as and when needed to cope with a virtually limitless number of option combinations, in practice the overall composite process is not responsive enough in terms of providing costing, testing and manufacturing system information in the required time scales.